This invention pertains to extended surface fins for heat exchanger tubes and a method of forming the same.
Existing helically wound, heat exchanger fins, as may be applied to a heat exchanger tube, are typically manufactured from relatively continuous strips of non-ferrous metal having a generally rectangular cross-sectional profile. The rectangular metal strips may be differentially rolled by passing the strips through a roll mill that applies a pressure differential across the width of the metal strips. The pressure differential is commonly applied by two rolls having forming faces that are angled one relative to the other.
The differential rolling process linearly stretches the strip to differing degrees across its width to force the strip into a cambered curl configuration having a reduced thickness at its outer diameter. The inside diameter of the cambered curl is wrapped helically, standing on edge, onto the outside diameter of a round heat exchanger tube. The tube is rotated and advanced in synchronization with the differential rolling process to produce a continuous, helically finned tube.
A method of forming an extended surface heat transfer fin consistent with the invention includes use of a tapered or wedge-shaped metal strip having a thickness along one edge greater than the thickness of the strip along the other, opposing edge. The wedge-shaped strip is differentially rolled, such that the initially thicker edge of the strip is compressed more than the thin edge of the strip, resulting a greater amount of linear stretching of the strip along the thicker edge. The differential rolling process produces a cambered curl structure that may be applied to a heat transfer tube to provide a helically wrapped extended surface heat transfer fin.